The term "polymer latex" is well known in the art and refers to an aqueous dispersion of a water-insoluble polymer which is present in the form of very fine particles. Polymer latices are often called aqueous emulsion polymers.
Polymer latices have found wide utility as intermediates for surface coating compositions. They are often employed as adhesives and as film forming agents in paint compositions designated for all types of applications.
Those skilled in the art have attempted to prepare polymers incorporating siloxane functionality by utilizing alkoxy silanes or alkoxy silane derivatives. U.S. Pat. No. 3,294,725 describes the aqueous emulsion polymerization of organosiloxanes and silcarbanes without using strong bases or strong mineral acids as the polymerization agent and without using a separate emulsifying agent. The '725 patent describes emulsion polymerization using a combined surface active sulfonic acid catalyst such as for example dodecylbenzene sulfonic acid (DBSA). However, the '725 patent is limited to the homopolymerization of organosiloxanes and silcarbanes, and the copolymerization of various types of organosiloxanes with each other or with silcarbanes.
U.S. Pat. No. 3,449,293 discloses the emulsion polymerization of organosilanes with unsaturated monomers, and more particularly of alkoxy silanes with acrylic esters to produce solid polymers. These solid polymers are disclosed as being insoluble in common organic solvents. This insolubility indicates that the polymers are substantially crosslinked. The solid polymers are disclosed as possessing improved thermal stability as compared with non-crosslinked polymers formed from the polymerization of corresponding unsaturated monomers without siloxane incorporation. The emulsion copolymerization mechanism is described as being a simultaneous addition and condensation reaction initiated using conventional water soluble free radical initiator of the peroxide type, a redox initiator system and emulsifier.
U.S. Pat. No. 3,575,910 is also directed to the preparation of siloxane-acrylate copolymers and aqueous emulsions containing these polymer particles. The copolymers contain 25 to 90 wt. percent acrylate and 10 to 75 wt. percent of a siloxane copolymer formed from 45 to 65 mole percent of D type (R.sub.2 SiO) monomer and 35 to 55 mole percent of T type (RSiO.sub.3/2) monomer. Preferably the siloxane-acrylate copolymer is formed by a two stage emulsion polymerization typically involving first forming the siloxane copolymer and secondly polymerizing the acrylate monomers in the presence of, and onto, the siloxane copolymer. An alternate method (example 13) in U.S. Pat. No. 3,575,910 is disclosed by which the acrylate and siloxane monomers are simultaneously polymerized using the stepwise addition of a free radical initiator and a buffer, but without the addition of a strong acid catalyst.
U.S. Pat. No. 3,706,697 discloses a free radical initiated aqueous emulsion polymerization of 55 to 90 percent by weight of an acrylic ester, from about 0.5 to 6 percent by weight of gamma-methacryloxypropyltrimethoxy silane (MATS) or gamma-acryloxypropyltrimethoxy silane and from about 9.5 to 44.5 weight percent of another copolymerizable free radical initiated monomer which does not have siloxane functionality.
U.S. Pat. No. 3,729,438 discloses emulsion polymers containing siloxane functionality formed from copolymers of vinyl acetate and a vinyl hydrolyzable silane, such as for example, MATS or vinyltrimethoxysilane (VTMS). The copolymers are disclosed as capable of post-crosslinking by means of the hydrolyzable siloxane functionality and the means disclosed to attempt to retard premature condensation crosslinking is through pH control of the aqueous emulsion within the range of pH 3.5 to 6.
Excessive premature crosslinking of siloxane-containing emulsion polymers was the problem addressed in Feasibility of Using Alkoxy Silane-Functional Monomers for the Development of Crosslinking Emulsions, T. R. Bourne, B. G. Bufkin, G. C. Wildman and J. R. Grave, Journal of Coatings Technology, Vol. 54, No. 684 January 1982. The authors acknowledge the inability to suppress the hydrolysis-condensation reaction of alkoxy silanes to acceptable levels despite optimizing reaction conditions to provide stable colloidal systems. In order to provide crosslinkable functionality with greater resistance to hydrolysis, the authors proposed using vinyl-type monomers with more sterically hindered alkoxy silane groups such as for example gamma-methacryloxy propylmethyldiethoxy silane (gamma-MAPMDES). However, because of the inability to prevent time dependent and implacable hydrolysis of the (alkyl--O--Si) bond in an aqueous environment, the authors concluded that the use of such sterically hindered alkoxy silane monomers, including gamma-MAPMDES, is limited mainly to applications requiring pre-crosslinked emulsion systems. The final conclusion of the paper was that if alkoxy-silane functional emulsions are to achieve the more ubiquitous status sought by industry for an advanced-generation system, then hydrolysis-resistant monomers or aqueous barrier techniques must be developed to prevent premature crosslinking of the alkoxy silane moiety.
Two references which rely on the crosslinking of siloxane moieties in emulsion copolymers are U.S. Pat. No. 3,898,300 and EPA 0153600. U.S. Pat. No. 3,898,300 discloses that the incorporation of crosslinked polyorganosiloxane particles into a styrenic copolymer matrix can improve impact strength to the polymer. EPA 0153600 discloses that emulsion polymerizing T (RSiO.sub.3/2) siloxanes with film forming monomers can provide coatings with crosslinked polyorganosiloxane microparticles which can act as rheology modifiers for solvent based formulations.
U.S. Pat. No. 5,214,095 teaches copolymers prepared by a concurrent free radical and cationic initiated emulsion polymerization of at least one free radical initiatable monomer, at least one linear siloxane precursor monomer, and at least one bifunctional silane monomer having both free radical polymerizable and silicon functional groups.
However, none of the aforementioned patents describe the particular method of preparing the novel emulsion polymers of this invention.